Halide solid electrolyte material and battery including the same

ABSTRACT

A solid electrolyte material according to the present disclosure is represented by the chemical formula Li 6-4a M a X 6 . M denotes at least one element selected from the group consisting of Zr, Hf, and Ti, X denotes at least one halogen element, and a is greater than 0 and less than 1.5.

BACKGROUND 1. Technical Field

The present disclosure relates to a halide solid electrolyte materialand a battery including the halide solid electrolyte material.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2011-129312discloses an all-solid-state battery containing a sulfide solidelectrolyte. Japanese Unexamined Patent Application Publication No.2006-244734 discloses an all-solid-state battery that includes anindium-containing halide as a solid electrolyte.

SUMMARY

One non-limiting and exemplary embodiment provides alithium-ion-conductive halide solid electrolyte material and a batteryincluding the halide solid electrolyte material.

In one general aspect, the techniques disclosed here feature a solidelectrolyte material represented by the chemical formula (I):Li_(6-4a)M_(a)X₆ (I), wherein M denotes at least one element selectedfrom the group consisting of Zr, Hf, and Ti, X denotes a halogenelement, and the following mathematical formula is satisfied: 0<a<1.5.

A battery according to the present disclosure includes a positiveelectrode, a negative electrode, and an electrolyte layer between thepositive electrode and the negative electrode, and at least one selectedfrom the group consisting of the positive electrode, the negativeelectrode, and the electrolyte layer contains the solid electrolytematerial.

The present disclosure provides a lithium-ion-conductive solidelectrolyte material and a battery including the solid electrolytematerial.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a battery according to a secondembodiment;

FIG. 2 is a schematic view of a press forming die used to measure theionic conductivity of a halide solid electrolyte material; and

FIG. 3 is a graph of a Cole-Cole plot of the impedance measurement of ahalide solid electrolyte material according to Example 1.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described below with referenceto the accompanying drawings.

First Embodiment

A halide solid electrolyte material according to a first embodiment isrepresented by the following chemical formula (I):

Li_(6-4a)M_(a)X₆  (I)

wherein

M denotes at least one element selected from the group consisting of Zr,Hf, and Ti,

X denotes at least one halogen element, and

the following mathematical formula is satisfied:

0<a<1.5.

The halide solid electrolyte material according to the first embodimenthas high lithium ion conductivity.

In the chemical formula (I), the mole fraction a of Li is equal to(6-4a). Likewise, the mole fraction_(R) of M is equal to a. In theactual analysis of the solid electrolyte material, α and β may have anerror of approximately 5% or less (desirably approximately 3% or less).The chemical formula (I) is satisfied with such an error.

In other words, the following two mathematical formulae may besatisfied:

0.95≤A/α≤1.05, and

0.95≤B/β≤1.05,

-   -   wherein

A and B denote the mole fractions of Li and M, respectively, determinedby actually analyzing the halide solid electrolyte material by ananalysis method, such as atomic absorption spectrometry or inductivelycoupled plasma emission spectrometry (hereinafter referred to as an“ICP-AES method”).

It is desirable that the following two mathematical formulae besatisfied:

0.97≤A/α≤1.03, and

0.97≤B/⊖≤1.03.

To increase the lithium ion conductivity of the halide solid electrolytematerial, the mathematical formula 0.01≤a≤1.45 may be satisfied. It isdesirable that the mathematical formula 0.5≤a≤1.1 may be satisfied. Itis more desirable that the mathematical formula 0.7≤a≤1.0 may besatisfied.

For example, the halide solid electrolyte material according to thefirst embodiment is represented by the chemical formula (II):Li_(6-4a)M_(a)Cl_(6-x-y)Br_(x)I_(y) (wherein 0≤x≤6, 0≤y≤6, and (x+y)≤6).Such a halide solid electrolyte material has high ion conductivity.

To further increase the lithium ion conductivity of the halide solidelectrolyte material, the mathematical formula (x+y)≤4 may be satisfied.It is desirable that the mathematical formula (x+y)≤1 be satisfied.

To further increase the lithium ion conductivity of the halide solidelectrolyte material, x may be less than 1.2. In other words, in thechemical formula (I), 80 atomic percent or more of X may be occupied byat least one element selected from the group consisting of Cl and I.

In the chemical formula (II), x may be equal to 0.

When y is greater than 0, the halide solid electrolyte material has highlithium ion conductivity. For example, y is 0.8 or more and 1.2 or less.

The halide solid electrolyte material according to the first embodimentcan be used to produce an all-solid-state battery with goodcharge-discharge characteristics. The all-solid-state battery may be aprimary battery or a secondary battery. The halide solid electrolytematerial according to the first embodiment can be used to produce asulfur-free all-solid-state battery. Even when the halide solidelectrolyte material according to the first embodiment is exposed to theatmosphere, no hydrogen sulfide is generated. Thus, an all-solid-statebattery including the halide solid electrolyte material according to thefirst embodiment is fairly safe. It should be noted that hydrogensulfide may be generated when a sulfide solid electrolyte disclosed inJapanese Unexamined Patent Application Publication No. 2011-129312 isexposed to the atmosphere.

The halide solid electrolyte material according to the first embodimentmay be crystalline or amorphous.

The halide solid electrolyte material according to the first embodimentis not limited to any particular shape. The shape may be acicular,spherical, or ellipsoidal. The halide solid electrolyte materialaccording to the first embodiment may be particles. The halide solidelectrolyte material according to the first embodiment may be formed ina pellet or sheet shape.

For example, when the halide solid electrolyte material according to thefirst embodiment is particulate (for example, spherical), the halidesolid electrolyte material according to the first embodiment may have amedian size of 0.1 micrometers or more and 100 micrometers or less. Themedian size means the particle size when the cumulative volume in thevolumetric particle size distribution is equal to 50%. The volumetricparticle size distribution can be measured with a laser diffractionmeasuring apparatus or an image analyzer.

To further increase the lithium ion conductivity of the halide solidelectrolyte material according to the first embodiment and to uniformlydisperse the halide solid electrolyte material according to the firstembodiment and an active material described later, the median size maybe 0.5 micrometers or more and 10 micrometers or less.

To further uniformly disperse the halide solid electrolyte materialaccording to the first embodiment and the active material, the halidesolid electrolyte material according to the first embodiment may have asmaller median size than the active material.

<Method for Producing Halide Solid Electrolyte Material>

The halide solid electrolyte material according to the first embodimentcan be produced by the following method.

First, two or more halide raw powders are mixed so as to have a desiredcomposition. A mixture is thus prepared.

For example, when the desired composition is Li₂ZrCl₆, a LiCl raw powderand a ZrCl₄ raw powder are mixed at a LiCl:ZrCl₄ mole ratio ofapproximately 2:1.

The raw powders may be mixed at a preadjusted mole ratio to offset apossible compositional change in the synthesis process described in thenext paragraph.

The raw powders in the mixture are mechanically reacted with each otherin a mixing apparatus, such as a planetary ball mill, (that is, by amechanochemical milling method) to obtain a reaction product. Thereaction product may be baked in a vacuum or in an inert atmosphere.Alternatively, the mixture may be baked in a vacuum or in an inertatmosphere to obtain a reaction product.

The baking may be performed at a temperature in the range of 100° C. to400° C. for 1 hour or more. To prevent the composition change caused bybaking, the reaction product or mixture may be put in an airtightcontainer, such as a quartz tube, before baking. A desired halide solidelectrolyte material is thus prepared.

Second Embodiment

A second embodiment is described below. The items described in the firstembodiment may be omitted.

An electrochemical device including the halide solid electrolyteaccording to the first embodiment is described in the second embodiment.

A battery is described below as an example of the electrochemical deviceaccording to the second embodiment. The battery according to the secondembodiment includes a positive electrode, a negative electrode, and anelectrolyte layer. The electrolyte layer is located between the positiveelectrode and the negative electrode. At least one selected from thegroup consisting of the positive electrode, the electrolyte layer, andthe negative electrode contains the halide solid electrolyte materialaccording to the first embodiment. The battery according to the secondembodiment has good charge-discharge characteristics. The battery may bean all-solid-state battery.

FIG. 1 is a cross-sectional view of a battery 1000 according to a secondembodiment. The battery 1000 includes a positive electrode 201, anelectrolyte layer 202, and a negative electrode 203. The positiveelectrode 201 contains positive-electrode active material particles 204and solid electrolyte particles 100. The electrolyte layer 202 islocated between the positive electrode 201 and the negative electrode203. The electrolyte layer 202 contains an electrolyte material (forexample, a halide solid electrolyte material). The negative electrode203 contains negative-electrode active material particles 205 and thesolid electrolyte particles 100.

The solid electrolyte particles 100 are composed of a halide solidelectrolyte material or composed mainly of a halide solid electrolytematerial.

The positive electrode 201 contains the positive-electrode activematerial particles 204, which can adsorb and desorb metal ions (forexample, lithium ions).

Examples of the positive-electrode active material includelithium-containing transition metal oxides (for example,LiNi_(1-d-f)Co_(d)Al_(f)O₂ (wherein 0<d, 0<f, and 0<(d+f)<1) andLiCoO₂), transition metal fluorides, polyanion materials, fluorinatedpolyanion materials, transition metal sulfides, transition metaloxyfluorides, transition metal oxysulfides, and transition metaloxynitrides.

To achieve a good dispersion state of the positive-electrode activematerial particles 204 and the solid electrolyte particles 100 in thepositive electrode 201, it is desirable that the positive-electrodeactive material particles 204 have a median size of 0.1 micrometers ormore. The good dispersion state improves the charge-dischargecharacteristics of the battery 1000. To rapidly diffuse lithium in thepositive-electrode active material particles 204, it is desirable thatthe positive-electrode active material particles 204 have a median sizeof 100 micrometers or less. Rapid diffusion of lithium enables thebattery 1000 to operate at high output. As described above, thepositive-electrode active material particles 204 may have a median sizeof 0.1 micrometers or more and 100 micrometers or less.

To easily achieve the good dispersion state of the positive-electrodeactive material particles 204 and the solid electrolyte particles 100,the positive-electrode active material particles 204 may have a largermedian size than the solid electrolyte particles 100.

From the perspective of the energy density and output of the battery,the ratio of the volume Vca1 of the positive-electrode active materialparticles 204 to the total of the volume Vca1 of the positive-electrodeactive material particles 204 and the volume Vce1 of the solidelectrolyte particles 100 in the positive electrode 201 may be 0.3 ormore and 0.95 or less. Briefly, the (Vca1)/(Vca1+Vce1)) ratio may be 0.3or more and 0.95 or less.

From the perspective of the energy density and output of the battery,the positive electrode 201 may have a thickness of 10 micrometers ormore and 500 micrometers or less.

The electrolyte layer 202 contains an electrolyte material. Theelectrolyte material may be the halide solid electrolyte materialaccording to the first embodiment. The electrolyte layer 202 may be asolid electrolyte layer. The electrolyte layer 202 typically has lithiumion conductivity but does not have electronic conductivity.

The electrolyte layer 202 may be composed only of the halide solidelectrolyte material according to the first embodiment. Alternatively,the electrolyte layer 202 may be composed only of a solid electrolytematerial different from the halide solid electrolyte material accordingto the first embodiment.

Examples of the solid electrolyte material different from the halidesolid electrolyte material according to the first embodiment includeLi₂MgX₄, Li₂FeX₄, Li(Al, Ga, In)X₄, Li₃(Al, Ga, In)X₆, and LiX. Xdenotes a halogen element (that is, at least one element selected fromthe group consisting of F, Cl, Br, and I).

The halide solid electrolyte material according to the first embodimentis hereinafter referred to as a first solid electrolyte material. Thesolid electrolyte material different from the halide solid electrolytematerial according to the first embodiment is referred to as a secondsolid electrolyte material.

The electrolyte layer 202 may contain not only the first solidelectrolyte material but also the second solid electrolyte material. Thefirst solid electrolyte material and the second solid electrolytematerial in the electrolyte layer 202 may be uniformly dispersed. Alayer formed of the first solid electrolyte material and a layer formedof the second solid electrolyte material may be stacked in the stackingdirection of the battery 1000.

From the perspective of the prevention of a short circuit between thepositive electrode 201 and the negative electrode 203 and the highoutput of the battery, the solid electrolyte layer may have a thicknessof 1 micrometer or more and 100 micrometers or less.

The negative electrode 203 contains the negative-electrode activematerial particles 205, which can adsorb and desorb metal ions (forexample, lithium ions).

Examples of the negative-electrode active material include metallicmaterials, carbon materials, oxides, nitrides, tin compounds, andsilicon compounds. The metallic materials may be single metals oralloys. Examples of the metallic materials include lithium metal andlithium alloys. Examples of the carbon material include naturalgraphite, coke, carbon during graphitization, carbon fiber, sphericalcarbon, artificial graphite, and amorphous carbon. From the perspectiveof capacity density, appropriate examples of the negative-electrodeactive material include silicon (that is, Si), tin (that is, Sn),silicon compounds, and tin compounds.

To achieve a good dispersion state of the negative-electrode activematerial particles 205 and the solid electrolyte particles 100 in thenegative electrode 203, the negative-electrode active material particles205 may have a median size of 0.1 micrometers or more. The gooddispersion state improves the charge-discharge characteristics of thebattery. To rapidly diffuse lithium in the negative-electrode activematerial particles 205, the negative-electrode active material particles205 may have a median size of 100 micrometers or less. Rapid diffusionof lithium enables the battery to operate at high output. As describedabove, the negative-electrode active material particles 205 may have amedian size of 0.1 micrometers or more and 100 micrometers or less.

To easily achieve the good dispersion state of the negative-electrodeactive material particles 205 and the solid electrolyte particles 100,the negative-electrode active material particles 205 may have a largermedian size than the solid electrolyte particles 100.

From the perspective of the energy density and output of the battery,the ratio of the volume vaa1 of the negative-electrode active materialparticles 205 to the total of the volume vaa1 of the negative-electrodeactive material particles 205 and the volume vae1 of the solidelectrolyte particles 100 in the negative electrode 203 may be 0.3 ormore and 0.95 or less. Briefly, the (vaa1)/(vaa1+vae1)) ratio may be 0.3or more and 0.95 or less.

From the perspective of the energy and output of the battery, thenegative electrode 203 may have a thickness of 10 micrometers or moreand 500 micrometers or less.

To increase ionic conductivity, chemical stability, and electrochemicalstability, at least one selected from the group consisting of thepositive electrode 201, the electrolyte layer 202, and the negativeelectrode 203 may contain the second solid electrolyte material.

As described above, the second solid electrolyte material may be ahalide solid electrolyte. Examples of the halide solid electrolyteinclude Li₂MgX₄, Li₂FeX₄, Li(Al, Ga, In)X₄, Li₃(Al, Ga, In)X₆, and LiX.X denotes a halogen element (that is, at least one element selected fromthe group consisting of F, Cl, Br, and I).

The second solid electrolyte material may be a sulfide solidelectrolyte.

Examples of the sulfide solid electrolytes include Li₂S—P₂S₅, Li₂S—SiS₂,Li₂S—B₂S₃, Li₂S—GeS₂, Li_(3.25)Ge_(0.25)P_(0.75)S₄, and Li₁₀GeP₂S₁₂.

The second solid electrolyte material may be an oxide solid electrolyte.

Examples of the oxide solid electrolyte include

-   -   (i) NASICON solid electrolytes, such as LiTi₂(PO₄)₃ and        element-substituted products thereof,    -   (ii) (LaLi)TiO₃-based perovskite solid electrolytes,    -   (iii) LIS ICON solid electrolytes, such as Li₁₄ZnGe₄O₁₆,        Li₄SiO₄, LiGeO₄, and element-substituted products thereof,    -   (iv) garnet solid electrolytes, such as Li₇La₃Zr₂O₁₂ and        element-substituted products thereof, and    -   (v) Li₃PO₄ and N-substitution products thereof.

The second solid electrolyte material may be an organic polymer solidelectrolyte.

Examples of the organic polymer solid electrolytes include compounds ofa polymer and a lithium salt. The polymer may have an ethylene oxidestructure. A polymer with an ethylene oxide structure can contain alarge amount of lithium salt and can have further increased ionicconductivity.

Examples of the lithium salt include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), andLiC(SO₂CF₃)₃. One lithium salt selected from these may be used alone.Alternatively, a mixture of two or more lithium salts selected fromthese may be used.

To facilitate lithium ion transfer and improve the outputcharacteristics of the battery 1000, at least one selected from thegroup consisting of the positive electrode 201, the negative electrode203, and the electrolyte layer 202 may contain a non-aqueous electrolytesolution, gel electrolyte, or ionic liquid.

The non-aqueous electrolyte solution contains a non-aqueous solvent anda lithium salt dissolved in the non-aqueous solvent.

Examples of the non-aqueous solvent include cyclic carbonate solvents,chain carbonate solvents, cyclic ether solvents, chain ether solvents,cyclic ester solvents, chain ester solvents, and fluorinated solvents.

Examples of the cyclic carbonate solvents include ethylene carbonate,propylene carbonate, and butylene carbonate.

Examples of the chain carbonate solvents include dimethyl carbonate,ethyl methyl carbonate, and diethyl carbonate.

Examples of the cyclic ether solvents include tetrahydrofuran,1,4-dioxane, and 1,3-dioxolane.

Examples of the chain ether solvents include 1,2-dimethoxyethane and1,2-diethoxyethane.

Examples of the cyclic ester solvents include γ-butyrolactone.

Examples of the chain ester solvents include methyl acetate.

Examples of the fluorinated solvents include fluoroethylene carbonate,methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate,and fluorodimethylene carbonate.

One non-aqueous solvent selected from these may be used alone, or amixture of two or more non-aqueous solvents selected from these may beused.

Examples of the lithium salt include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), andLiC(SO₂CF₃)₃.

One lithium salt selected from these may be used alone, or a mixture oftwo or more lithium salts selected from these may be used.

The concentration of the lithium salt is 0.5 mol/l or more and 2 mol/lor less, for example.

The gel electrolyte may be a polymer material impregned with anon-aqueous electrolyte solution. Examples of the polymer materialinclude poly(ethylene oxide), polyacrylonitrile, poly(vinylidenedifluoride), poly(methyl methacrylate), and polymers with an ethyleneoxide bond.

Examples of cations in the ionic liquid include

-   -   (i) aliphatic chain quaternary salts, such as tetraalkylammonium        and tetraalkylphosphonium,    -   (ii) alicyclic ammoniums, such as pyrrolidinium, morpholinium,        imidazolinium, tetrahydropyrimidinium, piperazinium, and        piperidinium, and    -   (iii) nitrogen-containing heteroaromatic cations, such as        pyridinium and imidazolium.

An anion in the ionic liquid may be PF₆ ⁻, BF₄ ⁻, SbF₆ ⁻, AsF₆ ⁻, SO₃CF₃⁻, N(SO₂CF₃)₂ ⁻, N(SO₂C₂F₅)₂ ⁻, N(SO₂CF₃)(SO₂C₄F₉)⁻, or C(SO₂CF₃)₃ ⁻.

The ionic liquid may contain a lithium salt.

To improve the adhesion between particles, at least one selected fromthe group consisting of the positive electrode 201, the negativeelectrode 203, and the electrolyte layer 202 may contain a binder.

Examples of the binder include poly(vinylidene difluoride),polytetrafluoroethylene, polyethylene, polypropylene, aramid resin,polyamide, polyimide, polyamideimide, polyacrylonitrile, poly(acrylicacid), poly(methyl acrylate), poly(ethyl acrylate), poly(hexylacrylate), poly(methacrylic acid), poly(methyl methacrylate), poly(ethylmethacrylate), poly(hexyl methacrylate), poly(vinyl acetate),polyvinylpyrrolidone, polyether, polyethersulfone,hexafluoropolypropylene, styrene-butadiene rubber, andcarboxymethylcellulose.

A copolymer may also be used as a binder. Examples of such a binderinclude copolymers of two or more materials selected from the groupconsisting of tetrafluoroethylene, hexafluoroethylene,hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride,chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene,fluoromethyl vinyl ether, acrylic acid, and hexadiene.

A mixture of two or more selected from these materials may be used as abinder.

To increase electronic conductivity, at least one selected from thepositive electrode 201 and the negative electrode 203 may contain aconductive aid.

Examples of the conductive aid include

-   -   (i) graphites, such as natural graphite and artificial graphite,    -   (ii) carbon blacks, such as acetylene black and Ketjen black,    -   (iii) electrically conductive fibers, such as carbon fiber and        metal fiber,    -   (iv) fluorocarbons,    -   (v) metal powders, such as an aluminum powder,    -   (vi) electrically conductive whiskers, such as zinc oxide        whiskers and potassium titanate whiskers,    -   (vii) electrically conductive metal oxides, such as titanium        oxide, and    -   (viii) electrically conductive polymers, such as polyaniline,        polypyrrole, and polythiophene.

With respect to the shape of the battery according to the secondembodiment, the battery is a coin battery, a cylindrical battery, arectangular battery, a sheet battery, a button battery (that is, abutton cell), a flat battery, or a laminated battery.

EXAMPLES

The present disclosure is described in detail in the following examples.

Example 1

[Preparation of Halide Solid Electrolyte Material]

A LiCl powder and a ZrCl₄ powder were prepared in an argon atmospherewith a dew point of −90° C. or less (hereinafter referred to as a dryargon atmosphere) such that the LiCl:ZrCl₄ mole ratio was 4.0:0.5. Thesepowders were ground and mixed in a mortar. A mixture was thus prepared.The mixture was then treated in a planetary ball mill at 600 rpm for 25hours for a mechanochemical reaction. A powder of the halide solidelectrolyte material according to Example 1 was thus prepared. Thehalide solid electrolyte material according to Example 1 had acomposition represented by Li₄Zr_(0.5)Cl₆.

The Li content per unit mass of the halide solid electrolyte materialaccording to Example 1 was measured by atomic absorption spectrometry.The Zr content of the halide solid electrolyte material according toExample 1 was measured by high-frequency inductively coupled plasmaspectroscopy. The Li:Zr mole ratio was calculated from the Li:Zrcontents determined by these measurements. The halide solid electrolytematerial according to Example 1 had a Li:Zr atomic ratio (that is, aLi:Zr mole ratio) of 4.0:0.5.

[Evaluation of Ion Conductivity]

FIG. 2 is a schematic view of a press forming die 300 used to measurethe ionic conductivity of a halide solid electrolyte material. The pressforming die 300 included a mold 301, a punch lower portion 302, and apunch upper portion 303. The mold 301 was formed of an insulatingpolycarbonate. The punch upper portion 303 and the punch lower portion302 were formed of an electrically conductive stainless steel.

The press forming die 300 illustrated in FIG. 2 was used to measure theionic conductivity of the halide solid electrolyte material according toExample 1 by the following method.

The press forming die 300 was filled with the powder of the halide solidelectrolyte material according to Example 1 in a dry argon atmosphere.

A pressure of 400 MPa was applied to the halide solid electrolytematerial according to Example 1 in the press forming die 300 via thepunch upper portion 303.

Under the pressure, the impedance of the halide solid electrolytematerial according to Example 1 was measured at room temperature by anelectrochemical impedance measurement method with a potentiostat(manufactured by Princeton Applied Research, trade name “VersaSTAT4”)via the punch lower portion 302 and the punch upper portion 303.

FIG. 3 is a graph of a Cole-Cole plot of the impedance measurement.

In FIG. 3, the real number of the complex impedance at a point ofmeasurement at which the absolute value of the phase of the compleximpedance was smallest was considered to be the resistance to the ionicconduction of the halide solid electrolyte material. For the realnumber, see the arrow R_(SE) in FIG. 3. The ionic conductivity wascalculated from the resistance using the following mathematical formula(III):

σ=(R _(SE) ×S/t)⁻¹  (III)

wherein

σ denotes the ionic conductivity,

S denotes the contact area between the solid electrolyte material andthe punch upper portion 303 (which is equal to the cross-sectional areaof the hollow portion in the mold 301 in FIG. 2),

R_(SE) denotes the resistance of the solid electrolyte material in theimpedance measurement, and

t denotes the thickness of the solid electrolyte material under thepressure (which is equal to the thickness of a layer formed of the solidelectrolyte particles 100 in FIG. 2).

The ionic conductivity of the halide solid electrolyte materialaccording to Example 1 measured at 22° C. was 1.1×10⁻⁴ S/cm.

Production of Secondary Battery

The powder of the halide solid electrolyte material according to Example1 and LiCoO₂ were prepared at a volume ratio of 30:70 in a dry argonatmosphere. These materials were mixed in an agate mortar to prepare amixture. The LiCoO₂ functioned as an active material.

The halide solid electrolyte material according to Example 1 (100 mg),the mixture (8.00 mg), and an aluminum metal powder (14.7 mg) weresuccessively layered in an insulating case with an inner diameter of 9.5millimeters to prepare a layered body. A pressure of 300 MPa was appliedto the layered body to form a first electrode and a solid electrolytelayer.

The solid electrolyte layer was brought into contact with a metal indiumfoil. The solid electrolyte layer was located between the metal indiumfoil and the first electrode. The metal indium foil had a thickness of200 micrometers. A pressure of 80 MPa was then applied to the metalindium foil. A second electrode formed of the metal indium foil was thusformed.

A stainless steel current collector was attached to the first electrodeand the second electrode, and a current collector lead was then attachedto the current collector. Finally, an insulating ferrule was used toisolate the inside of the insulating case from the outside airatmosphere and seal the insulating case. A secondary battery accordingto Example 1 was thus produced.

[Charge-Discharge Test]

The secondary battery was placed in a thermostat maintained at 25° C.

The battery according to Example 1 was charged at a current density of0.065 mA/cm² to a voltage of 3.6 volts. This current density correspondsto a rate of 0.05 C.

The battery according to Example 1 was then discharged at a currentdensity of 0.065 mA/cm² to a voltage of 1.9 volts.

The charge-discharge test showed that the secondary battery according toExample 1 had an initial discharge capacity of 264 μAh.

Examples 2 to 5

An experiment in Examples 2 to 5 was performed in the same manner as inExample 1 except that a LiCl powder and a ZrCl₄ powder were preparedsuch that the LiCl:ZrCl₄ mole ratio was (6-4a):m. Table 1 lists the aand m values.

Examples 6 to 10

An experiment in Examples 6 to 10 was performed in the same manner as inExample 1 except that the LiCl powder and a HfCl₄ powder were preparedsuch that the LiCl:HfCl₄ mole ratio was (6-4a):n. Table 1 lists the aand n values.

Examples 11 to 20

An experiment in Examples 11 to 20 was performed in the same manner asin Example 1 except that the LiCl powder, the ZrCl₄ powder, and theHfCl₄ powder were prepared such that the LiCl:ZrCl₄:HfCl₄ mole ratio was(6-4a):m:n. Table 1 lists the a, m, and n values.

Examples 21 to 23

An experiment in Examples 21 to 23 was performed in the same manner asin Example 1 except that the LiCl powder, the ZrCl₄ powder, and a ZrBr₄powder were prepared such that the LiCl:ZrCl₄:ZrBr₄ mole ratio was(6-4a):(m−x/4):(x/4). See Table 1 for the a, m, and x values.

Examples 24 to 26

An experiment in Examples 24 to 26 was performed in the same manner asin Example 1 except that the LiCl powder, the ZrCl₄ powder, and a ZrI₄powder were prepared such that the LiCl:ZrCl₄:ZrI₄ mole ratio was(6-4a):(m−y/4):(y/4). Table 1 lists the a, m, and y values.

Examples 27 to 29

An experiment in Examples 27 to 29 was performed in the same manner asin Example 1 except that the LiCl powder, the HfCl₄ powder, and a HfBr₄powder were prepared such that the LiCl:HfCl₄:HfBr₄ mole ratio was(6-4a):(n−x/4):(x/4). Table 1 lists the a, n, and x values.

Examples 30 to 32

An experiment in Examples 30 to 32 was performed in the same manner asin Example 1 except that the LiCl powder, the HfCl₄ powder, and a HfI₄powder were prepared such that the LiCl:HfCl₄:HfI₄ mole ratio was(6-4a):(n−y/4):(y/4). Table 1 lists the a, n, and y values.

Example 33

An experiment in Example 33 was performed in the same manner as inExample 1 except that a LiI powder, the ZrCl₄ powder, and the ZrBr₄powder were prepared such that the LiI:ZrBr₄:ZrCl₄ mole ratio was(6-4a):(m−x/4):x/4. Table 1 lists the a, m, and x values.

Example 34

An experiment in Example 34 was performed in the same manner as inExample 1 except that the LiI powder, the HfCl₄ powder, and the HfBr₄powder were prepared such that the LiI:HfCl₄:HfBr₄ mole ratio was(6-4a):(n−x/4):x/4. Table 1 lists the a, n, and x values.

Example 35

An experiment in Example 35 was performed in the same manner as inExample 1 except that the LiCl powder, the ZrCl₄ powder, and the TiI₄powder were prepared such that the LiCl:ZrCl₄:TiI₄ mole ratio was(6-4a):m:n. Table 1 lists the a, m, and n values.

The ionic conductivity of each halide solid electrolyte in Examples 2 to35 was measured in the same manner as in Example 1. Table 1 shows theresults.

Each halide solid electrolyte in Examples 2 to 35 was used to produce asecondary battery in the same manner as in Example 1. Like the batteryaccording to Example 1, the batteries according to Examples 2 to 35 hadgood charge-discharge characteristics.

Comparative Example 1

In Comparative Example 1, a LiBr powder and an InBr₃ powder wereprepared such that the LiBrInBr₃ mole ratio was 3:1. The powders wereground in a mortar to prepare a mixture. A pressure was applied to themixture to form a pellet. The pellet was put into a glass tube undervacuum and was then baked at 200° C. for 1 week. A solid electrolytematerial according to Comparative Example 1 was thus prepared. The solidelectrolyte material according to Comparative Example 1 had acomposition represented by Li₃InBr₆. The ionic conductivity of the solidelectrolyte material according to Comparative Example 1 was measured inthe same manner as in Example 1. The ionic conductivity measured at 22°C. was less than 1×10⁻⁷ S/cm.

Comparative Example 2

In Comparative Example 2, a LiCl powder and an FeCl₂ powder wereprepared such that the LiCl:FeCl₂ mole ratio was 2:1. The powders weremechanochemically mixed in the same manner as in Example 1. A solidelectrolyte material according to Comparative Example 2 was thusprepared. The solid electrolyte material according to ComparativeExample 2 had a composition represented by Li₂FeCl₄. The ionicconductivity of the solid electrolyte material according to ComparativeExample 1 was measured in the same manner as in Example 1. The ionicconductivity measured at 22° C. was 9×10⁻⁶ S/cm.

The solid electrolyte material according to Comparative Example 2 wasused to produce a secondary battery in the same manner as in Example 1.The secondary battery according to Comparative Example 2 was subjectedto a charge-discharge test. The secondary battery according toComparative Example 2 had an initial discharge capacity of 1 μAh orless. Thus, the secondary battery according to Comparative Example 2could not be charged or discharged. In other words, the secondarybattery according to Comparative Example 2 did not function as abattery.

Table 1 lists the ionic conductivities measured in Examples 1 to 35.Table 2 list the ionic conductivities measured in Comparative Examples 1and 2.

TABLE 1 Ionic conductivity Example Composition M a m n X x y (×10⁻⁴S/cm) 1 Li₄Zr_(0.5)Cl₆ Zr 0.5 0.5 — Cl — — 1.1 2 Li_(3.2)Zr_(0.7)Cl₆ Zr0.7 0.7 — Cl — — 3.2 3 Li_(2.4)Zr_(0.9)Cl₆ Zr 0.9 0.9 — Cl — — 4.9 4Li₂ZrCl₅ Zr 1 1 — Cl — — 4.7 5 Li_(1.6)Zr_(1.1)Cl₆ Zr 1.1 1.1 — Cl — —1.9 6 Li₄Hf_(0.5)Cl₆ Hf 0.5 — 0.5 Cl — — 1 7 Li_(3.2)Hf_(0.7)Cl₆ Hf 0.7— 0.7 Cl — — 2.1 8 Li_(2.4)Hf_(0.9)Cl₆ Hf 0.9 — 0.9 Cl — — 3.2 9Li₂HfCl₆ Hf 1 — 1 Cl — — 3.1 10 Li_(1.6)Hf_(1.1)Cl₆ Hf 1.1 — 1.1 Cl — —1.2 11 Li₄Zr_(0.25)Hf_(0.25)Cl₆ Zr, Hf 0.5 0.25 0.25 Cl — — 1.1 12Li_(3.2)Zr_(0.35)Hf_(0.35)Cl₆ Zr, Hf 0.7 0.35 0.35 Cl — — 2.2 13Li_(2.4)Zr_(0.45)Hf_(0.45)Cl₆ Zr, Hf 0.9 0.45 0.45 Cl — — 3.4 14Li₂Zr_(0.5)Hf_(0.5)Cl₆ Zr, Hf 1 0.5 0.5 Cl — — 3.3 15Li_(1.6)Zr_(0.55)Hf_(0.55)Cl₆ Zr, Hf 1.1 0.55 0.55 Cl — — 1.2 16Li₄Zr_(0.4)Hf_(0.1)Cl₆ Zr, Hf 0.5 0.4 0.1 Cl — — 1.1 17Li_(3.2)Zr_(0.56)Hf_(0.14)Cl₆ Zr, Hf 0.7 0.56 0.14 Cl — — 2.4 18Li_(2.4)Zr_(0.72)Hf_(0.18)Cl₆ Zr, Hf 0.9 0.72 0.18 Cl — — 3.7 19Li₂Zr_(0.8)Hf_(0.2)Cl₆ Zr, Hf 1 0.8 0.2 Cl — — 3.5 20Li_(1.6)Zr_(0.88)Hf_(0.22)Cl₆ Zr, Hf 1.1 0.88 0.22 Cl — — 1.4 21Li₂ZrCl₅Br Zr 1 1 — Cl, Br 1 — 2.5 22 Li₂ZrCl₄Br₂ Zr 1 1 — Cl, Br 2 —1.2 23 Li₂ZrCl₃Br₃ Zr 1 1 — Cl, Br 3 — 1 24 Li₂ZrCl₅I Zr 1 1 — Cl, I — 15.4 25 Li₂ZrCl₄I₂ Zr 1 1 — Cl, I — 2 1.8 26 Li₂ZrCl₃l₃ Zr 1 1 — Cl, I —3 1.1 27 Li₂HfCl₅Br Hf 1 — 1 Cl, Br 1 — 1.5 28 Li₂HfCl₄Br₂ Hf 1 — 1 Cl,Br 2 — 1.3 29 Li₂HfCl₃Br₃ Hf 1 — 1 Cl, Br 3 — 1 30 Li₂HfCl₅I Hf 1 — 1Cl, I — 1 3.2 31 Li₂HfCl₄I₂ Hf 1 — 1 Cl, I — 2 1.3 32 Li₂HfCI₃I₃ Hf 1 —1 Cl, I — 3 1.1 33 Li₂ZrCl₂Br₂I₂ Zr 1 1 — Cl, Br, I 2 2 1.1 34Li₂HfCl₂Br₂I₂ Hf 1 — 1 Cl, Br, I 2 2 1 35Li₂Zr_(0.83)Ti_(0.17)CI_(5.32)I_(0.68) Zr, Ti 1 0.83 0.17 Cl, I 5.320.68 2.5

TABLE 2 Comparative Ionic conductivity example Composition M X (S/cm) 1Li₃InBr₆ In Br less than 1 × 10⁻⁷ 2 Li₂FeCl₄ Fe Cl 9 × l0⁻⁶

As is clear from Tables 1 and 2, the halide solid electrolytes inExamples 1 to 35 have a high ionic conductivity of 1×10⁻⁴ S/cm or moreat room temperature. By contrast, the halide solid electrolytes inComparative Examples 1 and 2 have a low ionic conductivity of less than1×10⁻⁷ S/cm and 9×10⁻⁶ S/cm or less, respectively.

A comparison between Examples 2 to 4, 6 to 9, 11 to 14, 16 to 19, and 21to 35 and Examples 1, 5, 10, 15, and 20 shows that an a value of 0.7 ormore and 1.0 or less results in high ionic conductivity.

The batteries in Examples 1 to 35 could be charged and discharged atroom temperature. By contrast, the battery in Comparative Example 2could not be charged or discharged.

The halide solid electrolytes in Examples 1 to 35 contain no sulfur anddo not generate hydrogen sulfide.

Thus, a halide solid electrolyte material according to the presentdisclosure does not generate hydrogen sulfide, has high lithium ionconductivity, and is suitable for a battery that can be satisfactorilycharged and discharged.

A halide solid electrolyte material according to the present disclosureis used in electrochemical devices, such as batteries (for example,all-solid-state batteries).

What is claimed is:
 1. A halide solid electrolyte material representedby the following chemical formula (I):Li_(6-4a)M_(a)X₆  (I) wherein M denotes at least one element selectedfrom the group consisting of Zr, Hf, and Ti, X denotes at least onehalogen element, and the following mathematical formula is satisfied:0<a<1.5.
 2. The halide solid electrolyte material according to claim 1,wherein M contains at least one element selected from the groupconsisting of Zr and Hf.
 3. The halide solid electrolyte materialaccording to claim 1, wherein the following mathematical formula issatisfied:0.01≤a≤1.45.
 4. The halide solid electrolyte material according to claim3, wherein the following mathematical formula is satisfied:0.5≤a≤1.1.
 5. The halide solid electrolyte material according to claim4, wherein the following mathematical formula is satisfied:0.7≤a≤1.0.
 6. The halide solid electrolyte material according to claim1, wherein the halide solid electrolyte material is represented by thefollowing chemical formula (II):Li_(6-4a)M_(a)Cl_(6-x-y)Br_(x)I_(y)  (II) wherein the following threemathematical formulae are satisfied:0≤x≤6,0≤y≤6, and(x+y)≤6.
 7. The halide solid electrolyte material according to claim 6,wherein the following mathematical formula is satisfied:(x+y)≤4.
 8. The halide solid electrolyte material according to claim 7,wherein the following mathematical formula is satisfied:(x+y)≤1.
 9. A battery comprising: a positive electrode; a negativeelectrode; and an electrolyte layer between the positive electrode andthe negative electrode, wherein at least one selected from the groupconsisting of the positive electrode, the negative electrode, and theelectrolyte layer contains the halide solid electrolyte materialaccording to claim 1.